Sapphire windows generally exhibit extreme hardness, high thermal conductivity, and resistance to common chemical acids and alkalis. Chemically, sapphire is single crystal aluminum oxide (Al2O3). Synthetic sapphire can be produced industrially from agglomerated aluminum oxide, which is sintered and fused in an inert atmosphere. Sapphire is the second hardest crystal next to diamonds, and because of its structural strength, can be made much thinner than other common windows with the added benefit of improved transmittance.
Sapphire is often used for shatter resistant windows and various military body armor suits. Sapphire glass, which is highly transparent to wavelengths of light between 150 nm and 5500 nm, is crystalline sapphire used as an optical window or cover. Some windows are made from sapphire boules that have been grown in a specific crystal orientation, typically along the optical, or c-axis, for minimum birefringence. Sapphire boules are sliced into a desired window thickness and are polished to the desired surface finish. Sapphire windows are used in high pressure chambers for spectroscopy, crystals in various watches, and windows in grocery store barcode scanners. They are also used as end windows on high-powered laser tubes as their wide-band transparency and thermal conductivity allow them to handle very high power densities in the infrared and ultraviolet (UV) spectrum without degrading due to heating.
Sapphire glass is typically manufactured in panes up to 16 inches×18 inches×1 inch. Sapphire is limited in the dimensions that can be achieved in a single crystal. In general, there is an increasing need for larger format sapphire windows for applications such as naval technology. For instance, a destroyer is a fast maneuverable long-endurance warship intended to escort larger vessels in a fleet, convoy, or battle group and defend them against smaller powerful short-range attackers. Destroyers comprise Component Electro-optic Window Enclosures (CEWEs) having dimensions of, for instance, about 57″×39″. Similarly, infrared search and track (IRST) pods and joint strike fighter (JSF) surveillance windows have a need for large area format shaped windows.
Currently, multiple panes of sapphire can be tiled together using mullions—vertical elements that form a division between units of a window—to hold the panes in place. However, the mullions are typically made of metal and will scatter and obscure views of the optics located behind the sapphire windows. Glass frit bonding has also been investigated for holding panes together. The glass frit bonding procedure forms a thin glass mixture at the interface, which bonds the panes together. The window is only as strong as the glass phase holding the panes together, which may compromise the mechanical integrity of the window. Further, optical bonding, where the bonds are optically polished to a tenth wave flatness and then contacted and heat treated to form monolithic windows, has been employed. However, optical bonds are limited in scale to what can feasibly be ground and polished to a tenth wave flatness. Currently, a 13 inch bond length is the maximum.